function getXhr(){
    var xhr = null; 
	if(window.XMLHttpRequest) // Firefox et autres
	   xhr = new XMLHttpRequest(); 
	else if(window.ActiveXObject){ // Internet Explorer 
	   try {
	            xhr = new ActiveXObject("Msxml2.XMLHTTP");
	        } catch (e) {
	            xhr = new ActiveXObject("Microsoft.XMLHTTP");
	        }
	}
	else { // XMLHttpRequest non supporté par le navigateur 
	   alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest..."); 
	   xhr = false; 
	} 
	return xhr
}
jQuery.fn.center = function () {
  this.css("position","fixed");
  this.css("top", ( $(window).height() - this.height() ) / 2 + "px");
  this.css("left", ( $(window).width() - this.width() ) / 2 + "px");
  this.css("position","fixed");
  return this;
}